Khách hàng sử dụng các dịch vụ được cung cấp bởi một máy chủ. Máy chủ nhận được thông điệp yêu cầu từ khách hàng và xử lý chh đến máy chủ trong cáca mình hoặc bằng cách đi qua một con trỏ để mô tả một không gian địa chỉ của khách hàng, máy chủ truy cập bằng cách sử dụng chuyển dữ liệu hạt nhân qua trung gian. | 168 THE CLIENT-SERVER FRAMEWORK IN THEORY Why Have a Client-Server Framework A client makes use of services provided by a server. The server receives request messages from its clients and handles them either synchronously or Data is passed from the client to the server in the request message itself or by passing a pointer to a descriptor in the client address space which the server accesses using kernel-mediated data transfer. On Symbian OS servers are typically used to manage shared access to system resources and services. The use of a server is efficient since it can service multiple client sessions and can be accessed concurrently by clients running in separate threads. A server also protects the integrity of the system because it can ensure that resources are shared properly between clients and that all clients use the resources correctly. When the server runs in its own process it has a separate isolated address space and the only access a client has to the services in question is through a well-defined interface. By employing a server in a separate process the system can guarantee that misbehaved clients cannot corrupt any of the resources the server manages on its behalf or for other clients. On Symbian OS it is this rationale which protects the filesystem all access to file-based data must be made through the file server client or a higher-level component which uses it such as the stream store . Servers can also be used to provide asynchronous services because they run in a separate thread to their clients. Most of the system services on Symbian OS particularly those providing asynchronous functionality are provided using the client-server framework for example the window server for access to UI resources such as the screen and keypad the serial communications server for access to the serial ports and the telephony server. How Do the Client and Server Fit Together A Symbian OS server always runs in a separate .
